This invention relates generally to gas insulated equipment and more particularly to a gas insulated transmission line wherein the outer sheath is compartmentalized to minimize the possibility of fault interaction amongst the inner conductors disposed therein.
Gas-insulated transmission lines are being used on an ever-increasing scale in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, and higher power lines required by growing metropolitan areas and the growing demands for electrical energy. Compressed gas insulated transmission lines typically comprise a hollow sheath, a conductor within the sheath, a plurality of solid insulating spacers which support the conductor within the sheath, and a compressed gas such as sulphur hexafluoride to electrically insulate the conductor from the sheath. The typical assembly has been fabricated from relatively short sections of hollow cylindrical ducts or tubes in which the conductor and insulators are inserted. The assembly is usually completed in the factory, and the sections are welded or otherwise secured together in the field to form the transmission line. Gas barriers are provided at intervals along the length of the assembly, and, after evacuation of the line, an insulating gas is forced into the sheath under pressure.
In order for gas insulated transmission lines to be used for long lengths of transmission, it is necessary to reduce both the cost of manufacture and the cost of installation. One such manner of reducing the cost of transmission lines, for multiple phase power transmission, has been to place a plurality of electrical conductors within a single outer sheath. These multiple conductor lines, which typically have all three electrical phases disposed within a single hollow sheath, can reduce the cost of the transmission line by 10 or 15% as compared with isolated phase gas insulated transmission lines. This cost reduction is accomplished by, among others, the lower sheath cost, the necessity for less welding and narrower trenching for underground installation, and the lower power losses associated with the line.
Although such a three conductor system is less expensive than utilizing three independent conductors within three separate sheaths, the use of such plurality of conductors within the outer sheath creates problems which are not present within the single conductor lines. For example, in the multiple conductor systems, any single phase fault to ground or arcing between two of the conductors will very rapidly, if there is a massive fault current, result in a three-phase short circuit before the electrical current is interrupted. This three phase fault is much more severe on electrical equipment such as circuit breakers, transformers, or generators than a single phase to ground fault.
Additionally, in individual phase gas-insulated transmission systems, redundancy for the three phase line can be obtained by providing a fourth gas cable which, through suitable switching arrangements, can be energized in the unlikely event that one of the other phases of the gas cable fails. Though any failure may be unlikely, redundancy may be desirable for systems wherein an interruption can be economically disastrous. For an isolated phase system, this extra phase redundancy can be supplied at a cost increase of less than about 33% of the total cost, plus the cost of the switching arrangements. For the multiple conductor system, where any massive failure will incapacitate the entire cable, redundancy can be supplied only by adding a second multiple phase cable and hence by a cost increase of approximately 100%, plus the cost of the switching arrangement.